In the exploration of oil, gas and geothermal energy, drilling operations are used to create boreholes, or wells, in the earth. Drilling rigs used in subterranean exploration must be transported to the locations where drilling activity is to be commenced. These locations are often remotely located. The transportation of such rigs on state highways requires compliance with highway safety laws and clearance underneath bridges or inside tunnels. This requirement results in extensive disassembly of full-size drilling rigs to maintain a maximum transportable width and transportable height with further restrictions on maximum weight, number and spacing of axles, and overall load length and turning radius. These transportation constraints vary from state to state, as well as with terrain limitations. These constraints can limit the size and capacity of rigs that can be transported and used, conflicting with the subterranean requirements to drill deeper, or longer, and reach horizontal wells more quickly, requiring larger rigs.
Larger, higher capacity drilling rigs are needed for deeper (or horizontally longer) drilling operations, since the hook load for deeper operations is very high, requiring rigs to have a capacity of 500,000 lbs. and higher. Constructing longer, deeper wells requires increased torque, mud pump capacity and the use of larger diameter tubulars in longer strings. Larger equipment is required to handle these larger tubulars and longer strings. All of these considerations drive the demand for larger rigs.
Modern high capacity drilling rigs are transported in sections and reassembled at the desired location. Typical assemblies include a pair of top and bottom side boxes on either side of a center section to form the complete substructure. The mast is typically transported in three sections; the lower mast section, the center mast section, and the upper section. The top of the substructure forms the drill floor. The mast is assembled and raised above the drill floor. Then the substructure is raised, lifting the raised mast. The top sections of the substructure are connected to the center section at a location beneath the drill floor.
Larger rigs require a wider base structure for strength and wind stability, and this requirement conflicts with the transportability constraint and the time and cost of moving them. The wide leg base and large loads of the higher capacity drilling rigs impart higher outward forces at the drilling floor level where they are connected. The connection of the legs to the substructure is made at the top side boxes. The outward forces urge the side boxes away from the center section connected between them, pivoting at the connections beneath the drill floor level.
It remains desirable to prevent the separation that occurs between the top side boxes and the center sections on high capacity drilling rigs. It is desirable to accomplish this task affordably. It is also desirable to accomplish this task without significant modification to existing, conventional designs for drilling rigs. It is also desirable to accomplish this task without adding significant time to the rig-up and rig-down procedures. It is also desirable to accomplish this task without adding machine or crane work to the solution. It is also desirable to accomplish this task without having to engage manpower beneath the substructure.
In summary, the preferred embodiments of the present invention provide unique solutions to many of the problems arising from a series of overlapping design constraints, including design limitations, rig-up and rig-down, and cost constraints.